package com.ywb.edugui.utils;

/**
 * @author ywb
 * 雪花算法id生成器
 */

public class SnowflakeIdWorker {
    /** 开始时间截 (2020-01-01) */
    private static final long twepoch = 1577808000000L;
    /** 机器id所占的位数 */
    private static final long workerIdBits = 5L;
    /** 数据标识id所占的位数 */
    private static final long datacenterIdBits = 5L;
    /** 序列在id中占的位数 */
    private static final long sequenceBits = 12L;
    /** 机器ID向左移12位 */
    private static final long workerIdShift = sequenceBits;
    /** 数据标识id向左移17位(12+5) */
    private static final long datacenterIdShift = sequenceBits + workerIdBits;
    /** 时间截向左移22位(5+5+12) */
    private static final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    /** 工作机器ID(0~31) */
    private static long workerId = 0l;
    /** 数据中心ID(0~31) */
    private static long datacenterId = 0l;
    /** 毫秒内序列(0~4095) */
    private static long sequence = 0L;
    /** 上次生成ID的时间截 */
    private static long lastTimestamp = -1L;
    /** 生成序列的掩码，这里为4095 (0b111111111111=0xfff=4095) */
    private final static long sequenceMask = -1L ^ (-1L << sequenceBits);

    public SnowflakeIdWorker(long workerId, long datacenterId) {
//        传入工作ID和和数据中心
        this.workerId = workerId;
        this.datacenterId = datacenterId;
    }
    /**
     * 获得下一个ID (该方法是线程安全的)
     * @return SnowflakeId
     */
    public static synchronized String nextId() {
        long timestamp = System.currentTimeMillis();
        //如果当前时间小于上一次ID生成的时间戳，说明系统时钟回退过这个时候应当抛出异常
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(
                    String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        //如果是同一时间生成的，则进行毫秒内序列
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            //毫秒内序列溢出
            if (sequence == 0) {
                //阻塞到下一个毫秒,获得新的时间戳
                timestamp = System.currentTimeMillis();
            }
        }
        //时间戳改变，毫秒内序列重置
        else {
            sequence = 0L;
        }
        //上次生成ID的时间截
        lastTimestamp = timestamp;
        //移位并通过或运算拼到一起组成64位的ID
        return String.valueOf(((timestamp - twepoch) << timestampLeftShift) //
                | (datacenterId << datacenterIdShift) //
                | (workerId << workerIdShift) //
                | sequence);
    }

    public static void main(String[] args) {
        SnowflakeIdWorker idWorker = new SnowflakeIdWorker(0, 0);

        for (int i = 0; i < 100; i++) {
            String id = idWorker.nextId();
            System.out.println(id);
        }
    }
}
